1. Field of the Invention
The present invention relates to a polyester film for use in a capacitor. More particularly, the present invention relates to a biaxially oriented polyester film which has good breakdown strength and is suitable for use in a capacitor.
2. Description of the Related Art
Since poly-1,4-cyclohexanedimethylene terephthalate (hereinafter referred to as "PCT") has a higher melting point and a higher glass transition temperature than polyethylene terephthalate (hereinafter referred to as "PET"), a capacitor comprising a PCT film has stable electric characteristics in a wide temperature range, good dielectric strength at a high temperature and good resistance to hydrolysis, so that such capacitor has excellent durability even under high temperature and humidity conditions (see. Soc. Plastics Engrs. J., 17, 1083 (1961) and Insulation, April, 36 (1961)).
When break down strength of a PCT film having a small area of about 50 mm.sup.2 according the method defined by JIS C 2319, it is better than that of a PET film. However, when a capacitor comprising a PCT film with a large effective area of 10,000 mm.sup.2 according to the method defined by JIS C 2318 and its break down strength is measured, it is often lower than that of a PET film capacitor made by the same method. In general, as a capacitance of a capacitor increases, namely an effective area increases, the capacitor easily picks up insulation defects in the film so that a breakdown voltage of the capacitor decreases. The above fact that the break down strength of the PCT film and that of the PET film are reversed by the difference of the effective area may indicate that the PCT film possibly has more insulation defects than the PET film.
Though, as a film for a capacitor, some properties of the PCT film are much better than those of the PET film, the tendency of having more insulation defects is a large problem in using the PCT film in the capacitor.